The inventory protector feature ensures that your inventory is safe when running promotions.
If you are running % off coupon promotions without limiting the max order quantity of the product you are promoting you are putting your inventory at risk to unscrupulous buyers who can use the coupon code to buy multiple units.
For example, if you create a 80% off coupon, in order to get more sales momentum, a seller could buy your whole inventory at 80% off (if you didn't limit order quantity).
If you set an order quantity limit of 1-3 then you mitigate the risk of a malicious buyer clearing out your inventory.
The downside of order quantity limits is that a real customer is also limited so it can affect your cart size. That is why we recommend setting the quantity limit to 3 unless you are running a very high discount promotion (90%+).